Place sugar, lemon juice, zest, and butter, salt in a 2 quart sauce pan ( non- reactive or stainless steel ). Turn on your heat to low simmer and cook while stirring until the butter is melted and mixture is blended.
Whisk in the eggs and yolk ,blending the egg mixture into the butter mixture. Continue to stir constantly with a wooden spoon or spatula until the mixture thickens and coats the back of a spoon. (6-8 minutes.) Do not let the mixture boil at any time. A low simmer is all you want.
When curd thickens, take pot off the heat.
Then pour the lemon curd into a glass bowl or mason jar and place a piece of plastic wrap on top to prevent a skin from forming. Or not. Some people don’t care if a skin forms on top, and they just stir it in before using.
Refrigerate your curd for at least an hour before using or until it is nice and thick and cold. It will keep tightly covered in the refrigerator for a month! Well that is if you haven't eaten it all up the first few days. lol.
Right now I am making it to top my angel food cake. I top the cake with a dollop of lemon curd and a dollop of fresh whipped cream. OR I cut my cake into layers, fill the bottom layer with lemon curd and whipped cream and also use whipped cream on the top layer. So yummy.